The auto contrast optimization (ACO) is one of widely-used techniques for improving display qualities of panel display devices such as liquid crystal display devices. For example, contrast enhancement of a dark image under a situation in which the brightness of a backlight is desired to be reduced effectively suppresses deterioration of the image quality with a reduced power consumption of the liquid crystal display device. In one approach, the contrast enhancement may be achieved by performing a correction calculation on image data (which indicate grayscale levels of each subpixel of each pixel). Japanese Patent Gazette No. 4,198,720 B2 discloses a technique for achieving a contrast enhancement, for example.
An auto contrast enhancement is most typically achieved by analyzing image data of the entire image and performing a common correction calculation for all the pixels in the image on the basis of the analysis; however, according to an inventors' study, such auto contrast enhancement may cause a problem that, when a strong contrast enhancement is performed, the number of representable grayscale levels is reduced in dark and/or bright regions of images. A strong contrast enhancement potentially causes so-called “blocked up shadows” (that is, a phenomenon in which an image element originally to be displayed with a grayscale representation is undesirably displayed as a black region with a substantially-constant grayscale level) in a dark region in an image, and also potentially causes so-called “clipped white” in a bright region in an image.
One known approach to address such problem is local contrast correction. For example, Japanese Patent Application Publication No. 2001-245154 A discloses a local contrast correction. In the technique disclosed in this patent document, a small difference in the contrast between individual regions in the original image is maintained while the maximum difference in the contrast between the individual regions is restricted.
One known technique for a local contrast correction is to perform contrast correction of respective positions of tine image in response to the difference between the original image and an image obtained by applying low-pass filtering to image data. Such technology is disclosed, for example, in Japanese Patent Application Publications Nos. 2008-263475 A, H07-170428 A and 2008-511048 A. The technique using low-pass filtering, however, causes a problem of an increased circuit size, since this technique requires a memory for storing an image obtained by the low-pass filtering.
Another known technique for a local contrast correction is to perform a contrast correction of each area defined in the image of interest on the basis of the image characteristics of each area. Such technology is disclosed, for example, in Japanese Patent Application Publications Nos. 2001-113754 A and 2010-278937 A. In the technique disclosed in these patent document, a contrast correction suitable for each area is achieved by setting the input-output relation of input image data and corrected image data (image data obtained by performing contrast correction on the input image data) for pixels of each area on the basis of the image characteristics of each area.
The technique which performs a contrast correction of each area defined in the image on the basic of the image characteristics of each area may undesirably cause discontinuities in the displayed image at boundaries between adjacent areas. Such discontinuities in the displayed image may be undesirably observed as block noise.
In the technique disclosed in Japanese Patent Application Publications No. 2010-278937 A, the input-output relation of input image data and corrected image data is continuously modified to resolve such discontinuities in the displayed image (refer to FIG. 1). This technique, however, may undesirably cause a halo effect when an image including a constant-color region near an image edge (for example, an image including a display window) is displayed.
FIG. 1 is a conceptual diagram illustrating an example of the halo effect. FIG. 1 illustrates an example of occurrence of a halo effect in a technique in which the gamma value of a gamma curve used for contrast correction is determined on the basis of the average picture level (APL) of each area. It should be noted that the gamma curve is a curve specifying the input-output relation between input image data and corrected image data.
For example, let us consider the case when input image data of an image including a first region of a constant color with a luminance value of 200 and a second region of a constant color with a luminance value of 20 are provided and areas arrayed in two rows and two columns are defined in the image, and the APLs of the areas are calculated as 155, 110, 110 and 20, respectively, as illustrated in FIG. 1.
When a gamma value of γA is determined with respect to position A in the area with an APL of 150 and a gamma value of γB is determined with respect to position B in an area with an APL of 110, the gamma value is determined so as to continuously modified between positions A and B with the technique in which the input-output relation between the input image data and the corrected image data is continuously modified; however, the continuous modification of the gamma value results in that the finally-obtained grayscale levels of the respective colors indicated in the corrected image data are different even if the input image data indicates the constant grayscale levels of the respective colors. This is undesirably observed as a halo effect.
FIG. 2 schematically illustrates an image which experiences a halo effect. Let use consider the case when the original image (illustrated in FIG. 2(a)) is an image in which a rectangular window 102 with a constant color is superposed on a background 101 with a constant color. In this case, it would be desirable that the image obtained by the contrast correction (FIG. 2(b)) is also displayed as an image in which the rectangular window 102 with a constant color is superposed on the background 101 with a constant color; however, the use of the technique in which the input-output relation between the input image data and the corrected image data is continuously modified undesirably results in that an halo effect is observed in which a gradation occurs near the edges of the rectangular window 102, as illustrated in FIG. 2(c).
As thus discussed, there is a need for providing a technique which effectively reduces a discontinuity in the display region at edges of areas in a contrast correction based on the image characteristics of respective areas defined in the image, while suppressing occurrence of a halo effect.